The present invention relates to the fabrication and processing of plastic film, and is more specifically directed to stabilizers for maintaining a tubular extrusion of blown plastic film, that is, a bubble, on a predetermined path during the extrusion process. The invention is more particularly concerned with improvements to an external guide or stabilizer arrangement that permits the aperture of the stabilizer to be adjusted, and which minimizes or eliminates problems caused by friction or film distortion as the extruded thermoplastic material is drawn through the guide stabilizer or sizing cage.
Polyethylene film or films of other thermoplastic materials are produced using a blown film process, in which molten thermoplastic material is fed to an annular extrusion die and the latter produces a tubular extrusion, i.e. bubble, of the material, which is drawn upwards and solidifies into film. An air jet incorporated in the die injects air into the interior of the extrusion to inflate the bubble or tube. The film in the extrusion is initially somewhat fluid, and inflates to a larger diameter, with a correspondingly thinner wall, cooling and solidifying when a predetermined thickness is reached. The process is controlled and adjusted so that the film has a uniform thickness, and this basic process is well known in the art. The tubular extrusion is drawn upwards and flattened between a pair of rollers, i.e., nip rollers, that are positioned above the die. Typically, a collapsing frame is provided to urge opposite sides of the tubular extrusion towards one another just before the tube reaches the nip rollers. The collapsing frame may consist of a pair of opposed arrays of horizontal collapsing boards of any of a number of well-known configurations, and one arrangement is discussed in U.S. Pat. No. 4,943,226, granted Jul. 24, 1990. The film exits the nip rollers as multiple-thicknesses of film, and the film proceeds from there to various cutting, printing, rolling, or other equipment.
As mentioned above, it is important that the thickness, as well as strength and other properties, of the produced film be as uniform as possible. It is also necessary to the quality of the film that it be relatively free of weak regions and tears. For those reasons, stabilizer arrangements are used to keep the tubular extrusion confined to a well-defined travel path as it proceeds from the die to the nip rollers. Guide cages or other external stabilizers can be positioned outside the bubble for this purpose. In some cases internal guide members can disposed atop the extrusion die and within the bubble, both to prevent the tubular extrusion from drifting off the axis of travel, and also to assist in the direction of air flow within the bubble from the die.
At the current time, an external stabilizer takes the form of a series of banks of bowed arms positioned at intervals around the tubular extrusion. In one arrangement that is often used, each arm carries a row of rollers made of PTFE (Teflon). The arms are positioned just out of contact with the bubble, or else in light contact. When the bubble or extrusion drifts laterally, i.e., away from its vertical axis, the film contacts the arms and rollers on that side. As the extrusion moves past the arms, the rollers turn, and also push the extrusion back towards the vertical axis. Unfortunately, the extruded plastic is highly abrasive, so that when the bubble moves vertically past the rollers at high speed, it tends to wear the rollers flat on one side. Also, because the rollers are positioned along a curve, the adjacent rollers touch on the bubble side, but are spaced from one another on the outer side of the arms. Therefore, there is a tendency for the rollers to pinch the film as it moves past, creating snags and tears. This is especially the case where the rollers exhibit wear and so that material has eroded away. Furthermore, it is difficult and expensive to replace worn PTFE rollers, and replacement typically involves replacing all the rollers along the entire bowed arm. In addition, the materials and construction involved make the external stabilizer of this design rather expensive.
While some previous constructions of bubble stabilizers have employed an array of arms configured as a variable iris aperture, these have employed bowed arms of rollers to guide the extruded tube. One example of this construction is shown in U.S. Pat. No. 4,815,957. The external stabilizers of this type are complicated and are difficult to maintain.
Another external stabilizer is described in U.S. Pat. No. 5,700,489, in which the stabilizer includes non-rotating bow arms of light-weight aluminum and a wear cover made of a wear resistant low-friction plastic material. There can be either a bank of curved bow arms or a spiral or helix member. This construction avoids the problems of the rollers as discussed above. However, there is no provision for easy adjustment of the aperture size or for creating an aperture that varies in radius from bottom to top.
A recognized problem in the blown film art is that blown film has different stretch and strength characteristics in the axial or machine direction from what is achieved in the transverse direction, i.e., circumferentially around the extrusion. This occurs because is has been difficult to control the expansion of the bubble in both directions at the same time. Ideally, the strength in the machine direction, or MD, should be the same as the strength in the transverse direction, or TD. Stabilizing the position of the extrusion or bubble would help in this regard. Also, controlling rotation of the bubble as it rises will affect these properties. However film materials of different composition, and films of different thickness may require more or less rotation, so some means should be provided to change the amount of rotation that the stabilizer induces onto the extruded bubble.
Also, previous stabilizer designs did not provide much attention to heat management, and did not control the cooling of the extruded bubble as it passed through the aperture of the sizing cage. In the case of the traditional roller-arm design, it is impossible to provide a flow of water or other coolant through the parts of the sizing cage that contact the film.